1. Field
One embodiment of the invention relates to a storage medium such as a recordable optical disc, a recording method, and a reproducing apparatus.
2. Description of the Related Art
As recording media capable of recording a large quantity of information such as video signals, digital versatile discs (DVDs) have been popularized. Consequently, a movie of about two hours is recorded on a DVD, and information is reproduced by a reproducing apparatus, which makes it possible to freely watch the movie at home. In recent years, digitization of television broadcasting has been proposed, and a plan has been made to put a high-resolution television system which is called a high-definition television (HDTV) system to practical use. For that purpose, a standard for a next-generation DVD has been proposed in which a recording capacity is increased by narrowing down a beam spot, for example, in such a manner that a wavelength of a laser beam is shortened, or a numerical aperture NA is enlarged. As a technique of increasing a recording capacity, use of a single-sided multilayer recording medium has been considered in addition to the method of narrowing down a beam spot. The single-sided multilayer recording medium is configured such that a plurality of recording layers are provided on one side of a disc, and a beam is focused on the respective layers by moving an objective lens in an optical axis direction, which makes it possible to write/read for each recording layer (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2004-206849, paragraphs 0036 to 0041, FIG. 1)
A single-sided multilayer information recording medium has the problem of interlayer crosstalk which is not generated in a single-sided single-layer recording medium. For ease of explanation, dual layers will be described as an example. In a single-sided dual layer recording medium, a laser beam is focused on the respective layers from a single read surface. A layer close to the read surface is called Layer 0, and a layer distant from the read surface is called Layer 1. When a beam is focused on each layer, some laser beam is irradiated onto a layer other than a target layer. For this reason, a reflected light from the layer other than the target layer is mixed up with a reproduction signal in reproduction, which brings about interlayer crosstalk. Note that interlayer crosstalk could be a problem in, not only reproduction, but also recording.